As is previously known, many metallurgical processes require the injection of an inert gas or gases, such as Argon, into a molten metal while the metal is being held within a refractory lined ladle. This provides a secondary treatment or refinement process prior to transporting the metal to a continous caster or teaming isle for casting into a solid shape. This secondary treatment, following decarburization of the liquid metal (the iron being converted into steel through removal of impurities in a basic oxygen furnace vessel, or other like converter), is accomplished by using an externally lined refractory pipe lance or other nozzle. Examples of pipe lances of this type are shown in U.S. Pat. No. 4,854,553 and U.S. Pat. No. 4,367,868.
A pipe lance produces bubbles by injecting the gas into the molten metal under pressure for a variety of purposes. The bubbling pipe lance serves the purposes of (1) temperature control, composition adjustment, and the ejection of impurities from the metal up into the slag, (2) the addition of nitrogen gas, and (3) the addition of oxygen for secondary decarburization or temperature adjustment.
Present refractory gas injection pipe lances have three general types of construction which allow a gas to be dispersed in the molten metal: (a) an open single pipe, or plurality of exit pipes, attached to a center pipe which protrude through a refractory lining at the bottom of the lance with direct contact to the metal, (2) a porous body with a performed shape of a permeable refractory matrix (permeability 0.1 to greater than 1.0 cm.sup.3 -cm/sec-cm.sup.2 -g/cm.sup.2, in many cases) connected to the lower lance pipe center on the side and the bottom, and (3) a metal pipe, a metal tubular pipe, or a conical metal spinning, any of which can contain a porous refractory plug which allows the passage of gases in sufficient quantities so as to produce the desired process control in the metal bath.
During the bubbling or stirring of a molten metal bath in a transport ladle using the above designs, premature failure of the lance tip is very common such that the full useful life of the total lance is not realized. By lance tip, what is meant is the lower 12" to 16" of the submerged end of the pipe lance through which the gases are expelled into the liquid metal bath. The high temperatures, the caustic slag, and the abrasion caused by stirring all tend to combine for a hostile environment for the lance.
With lances which have a single or multiple pipe discharge ports, low gas velocity causes large bubbles which are unable to force the molten metal away from the lance thereby causing accelerated refractory erosion and premature lance tip failure. Further, the melting and collapse of the exit pipes causes rapid deterioration of the pipe lance as the entire gas flow becomes uncontrolled or stopped.
With a porous body having a performed shape of refractory matrix, higher velocity gas discharges are realized than with the pipe method and the gas is ejected in the form of small bubbles over a greater surface area. The higher gas velocity and smaller bubbles are more protective of the lance. Howewver, there are other problems associated with the performed porous body. These systems must discard the good physical properties associated with a low porosity-high density ceramic which is designed for extended submerged contact with liquid steel at temperatures between 2,820.degree. F. to 3,150.degree. F. for their high permeability. The physical properties of the low porosity-high density ceramic, which are sacrificed in this tradeoff are high erosion resitance, high corrosion resistance, high abrasion resistance against the severe molten slag and steel stirring, density, and physical strength. Because of its higher porosity, the porous body has very poor volume stability, and, thus a high shrinkage which causes the porous body to wear quickly and, thus, prematurely.
With a porous plug sheathed within a cylindrical or conical metal casing, various blends of refractory aggregates are used (tubular alumina, calcined alumina, fused alumina, mullite, chromic oxide, chromite, quartz, magnesite, synthetic alumina-silicates, zircon, etc.) to produce a porous plug similar in physical and chemical properties to the porous performed shape. However, the same loss in physical properties occurs in this type of permeable ceramic plug as in the preshaped structure. Premature wear of the porous plug can cause the plug to be blown out of a lance or nozzle refractory wall altogether. This type of failure causes an immediate reduction of gas velocity and consequent accelerated wear and premature failure of the lance tip slide wall above the port which mounts the porous plug.
With these structures, the flow rate of a pipe lance is a direct function of the permeability of the porous refractory body. As indicated in ASTM (American Standard Testing Methods) Part 13, ASTM Designation: C 577-87, "Test Method For The Permeability Of A Refractory" the permeability of a refractory body is proportional to the length of the specimen. Thus, as the lance tip wears and the refractory porous media becomes less thick, the flow rate increases linearly at a constant flow pressure. This activity results in an unbalanced and uncontrolled system because there is no method of readily controlling the rate of erosion or knowing what it is.
Therefore, the prior bubbling refractory pipe lances described do not result in optimum overall useful life of the gas injection refractory pipe lance, due in many cases, to the failure of the lance tip because of the premature failure of metal pipes, a porous plug and/or the surrounding refractory tip material. Furthermore, no flow and low or uncontrolled flow rates are a continuous problem with these lances resulting in the removal of the pipe lance from operation.